Hans Kraus

Abstract:

The CRESST-II direct Dark Matter search is located in the Gran Sasso underground laboratories, Italy. CaWO4 crystals are used as scintillating targets for WIMP (weakly interacting massive particle) interactions. They are operated as cryogenic calorimeters in combination with a second cryogenic detector used to measure the scintillation light produced in the target crystal. For each particle interaction, the combination of phonon and light signals provides an event by event discrimination which allows to distinguish known particles (alphas, betas, gammas, neutrons) from the expected signal of WIMPs. A major upgrade of the setup comprises modifications of the shielding, installation of a muon-veto, and new read out electronics, as well as a new detector-support structure to accommodate up to 33 detector modules, i.e. 10 kg of target mass. The experiment was thereafter successfully commissioned in 2007. Data obtained during this commissioning phase from 2 detector modules are presented here. Combining the data collected with these two detector modules with data from one single module obtained during the CRESST-I phase, the experiment could already place a limit of ∼6 × 10-7 pb for the spin independent WIMP-nucleon scattering cross section at a WIMP mass of ∼60 GeV/c2. © BAS, BDP Sciences 2009.

Abstract:

Results are presented on our latest research, aimed at the development and study of oxide scintillation crystals (ZnWO4, ZnMoO4, PbWO4, PbMoO4, and MgWO4) with high scintillation yield and low intrinsic radioactivity. We report on the improvement of these properties for conventional scintillators, as well as on new promising crystals based on metal tungstates and molybdates. The results are discussed in view of applying these materials in cryogenic experiments searching for dark matter and/or neutrinoless double beta decay. © 2006 IEEE.

Abstract:

In order to optimize sapphire as a cryogenic scintillation-phonon detector for dark matter, Al2 O3 : Ti crystals with different concentrations of doping have been studied using continuous X-ray excitation in the 30-300 K temperature range. Light yields vary by 20% for Ti concentrations between 10 and 1000 ppm at room temperature; they roughly double as the crystals are cooled from room temperature to 45 K. From the analysis of the change in the X-ray luminescence spectra of Al2 O3 with the concentration of Ti, it is concluded that the well-known blue emission of Ti-doped Al2 O3 is due to the radiative decay of F-centers. Recommendations are given for improving the performance of Al2 O3 scintillators. © 2009 Elsevier B.V. All rights reserved.

Abstract:

Large volume zinc tungstate crystal scintillators with improved scintillation characteristics were produced. Energy resolution and relative light output of samples with various sizes and shapes (cylinders, rectangular, and hexagonal prisms) were investigated. The energy resolutions of ZnWO 4 scintillator modules for the 662 keVγ line of 137Cs were 8.5% for a 1 cm3 sample and 10.7% for the large hexagonal module of dimensions ◇ 40 × 40 mm. A variation of the light output by 7%, depending on direction of observation was found for the 1 cm3 cubic crystal. Afterglow,measured 20 ms after termination of irradiation, was 0.002%. The relative light output of the scintillator and the decay kinetics were studied over the temperature range 7-300 K. The level of radioactive contamination of the ZnWO4 scintillator was determined in the Solotvina Underground Laboratory at a sensitivity of ≈0.1-10 mBq/kg. © 2006 IEEE.